1. Field of the Invention
The present invention relates to a process for the detection of the rectifier effect appearing in a gas discharge lamp and to an electronic ballast, for the operation of at least one gas discharge lamp, with the aid of which a rectifier effect appearing in the gas discharge lamp can be detected.
Gas discharge lamps are, as is known, operated with the aid of so-called electronic ballasts.
Such an electronic ballast is known for example from EP-B1-0 338 109. FIG. 10 shows the basic structure of this electronic ballast.
2. Description of the Related Art
The electronic ballast shown in FIG. 10 includes first a circuit A which is connected to the a.c. mains. This circuit A serves as HF-harmonics filter for reducing the higher-order harmonics of the mains frequency and for elimination of radio interference.
A rectifier circuit B is connected to the circuit A, which rectifier circuit transforms the mains voltage into a rectified intermediate voltage and supplies this via a harmonics filter C, which serves for smoothing the intermediate voltage, to an inverter circuit D. This inverter circuit D serves quasi as controllable a.c. voltage source and converts the d.c. voltage of the rectifier B into a variable a.c. voltage. The inverter D includes as a rule two (not shown) controllable switches, for example MOS field effect transistors. The two switches are connected in the form of a half-bridge circuit and are so alternatingly controlled with the aid of a corresponding bridge driver that in each case one of the switches is switched on and the other switched off. The two inverter switches are, thereby, connected in series between a supply voltage and ground, whereby at the common node between the two inverter switches a load circuit or output circuit E is connected, in which a gas discharge lamp or fluorescent lamp G is arranged. This output circuit E includes a series resonance circuit via which the xe2x80x9cchoppedxe2x80x9d high frequency a.c. voltage of the inverter D is supplied to the fluorescent lamp G.
Before the application of the ignition voltage to the fluorescent lamp G, the lamp electrodes of the fluorescent lamp G are pre-heated, in order to extend the lifetime of the lamp. The pre-heating can be effected for example with the aid of a heating transformer the primary winding of which is connected with the series resonance circuit, whereas the secondary winding of the heating transformer is coupled with the individual lamp coils. In this way it is possible to supply the lamp coils with energy also in ignited operation. In pre-heating operation, the frequency of the a.c. voltage delivered from the inverter D is so altered, with regard to the resonance frequency of the series resonance circuit of the output circuit E, that the voltage applied to the gas discharge lamp G does not cause ignition of the lamp. In this case there flows through the lamp electrodes of the lamp, in the form of coils, a substantially constant current by means of which the lamp coils are pre-heated. After conclusion of the pre-heating phase the frequency of the a.c. voltage delivered from inverter D is displaced into the vicinity of the resonance frequency of the series resonance circuit, whereby the voltage applied to the gas discharge lamp G increases so that the gas discharge lamp G is ignited.
During the pre-heating, ignition and operation of the gas discharge lamp G, certain fault conditions can appear which are to be identified in order to be able to appropriately react thereto. For this purpose, the electronic ballast has a control circuit F which monitors various circuitry parameters of the electronic ballast and upon a limit value being exceeded generates a corresponding control signal for the inverter D in order to alter the frequency of the a.c. voltage generated from the inverter D in dependence upon the detected fault condition. Thus, for example, the control circuit F can monitor the lamp voltage, the pre-heating voltage, the lamp operating current, the impedance phase angle of the output circuit E or the d.c. voltage generated from the rectifier B and can so set the inverter frequency that the lamp voltage, the pre-heating voltage or the lamp current do not exceed a predetermined limit value, the d.c. power taken from the rectifier B is as constant as possible, or a capacitive operation of the series resonance output circuit E is avoided.
As also in the case of other lamps, with gas discharge lamps there appears, as a result of wear manifestations of the heating coils, at the end of the lifetime of the gas discharge lamp, the effect that the lamp electrodes wear out unevenly with time, i.e. the degradation of the emission layers on the lamp electrodes is different. Due to this different wear of the lamp electrodes there arise differences in the emission capabilities of the two lamp electrodes.
This difference in emission capabilities has the consequence that in the gas discharge lamp concerned there flows from the one lamp electrode to the other a higher current than vice versa, so that the temporal development of the lamp current exhibits an excess during one half-wave. Due to the different degradation of the two lamp electrodes there thus come about asymmetries which bring about not only a strong light flickering at the end of the lifetime of the gas discharge lamp, but in the extreme case allow an operation of the gas discharge lamp only during one half-wave, i.e. during the excessive half-wave. The gas discharge lamp acts in the same way as a rectifier, so that the above-described effect is called the xe2x80x9crectifier effectxe2x80x9d.
At that lamp electrode which, with time, has worn more strongly, the emission work function of the electrons is greater than at the less strongly worn electrode. As emission work function there is generally meant the minimum energy which is needed to remove an electron from a metal, in this case from the lamp electrode. The dipole layer at the surface of the metal, i.e of the lamp electrode, is thereby an important factor for defining the emission work function. The more strongly worn lamp electrode, having a greater emission work function for the electrons, as a consequence heats up more strongly than the less strongly worn electrode upon putting into operation the gas discharge lamp. The heating of the lamp electrode can in particular with lamps of having small diameter be so great that parts of the lamp glass bulb may even melt. In order to avoid the danger of accident resulting from the heating of the lamp glass bulb during the operation of the gas discharge lamp, the rectifier effect must consequently be recognised and, if appropriate, the gas discharge lamp switched off or its power take-up reduced.
In this regard, it is already known to detect the appearance of a rectifier effect by monitoring the lamp current flowing through the gas discharge path of the lamp. With the aid of this process there can be directly recognised emission differences of the lamp electrodes, but the evaluation of these emission differences and the realisation of this recognition process in a monitoring circuit configured as an integrated circuit is problematic. Alternatively, the rectifier effect can be recognised also by means of monitoring of the peak value of the lamp voltage since the asymmetries appearing in the lamp current are carried over to the lamp voltage. If, for example, the lamp voltage exceeds a particular limit value as a result of the asymmetric emission of the lamp electrodes, the gas discharge lamp is automatically switched off. With this recognition process it is however disadvantageous that the sensitivity of the process is very limited since in the case of a fault, i.e. upon appearance of the rectifier effect, the peak value of the detected lamp voltage is only 60% higher than in normal operation. Further, upon dimming of the gas discharge lamp, the lamp voltage changes so that it may occur that upon dimming of the gas discharge lamp there is erroneously determined the presence of the rectifier effect as a result of the thereby increased lamp voltage. Overall, the detection of the rectifier effect with the aid of a monitoring of the peak value of the lamp voltage is thus problematic.
The present invention is thus based on the object of proposing a possibility for the detection of the rectifier effect appearing in a gas discharge lamp such that the rectifier effect can be detected more simply and in particular more precisely.
In accordance with the invention this object is achieved by means of a process described hereinafter and a corresponding electronic ballast also described hereinafter.
In accordance with the present invention it is proposed to detect the lamp voltage, or a parameter dependent thereupon, but in accordance with the present invention the detected parameter is integrated and then the integration result evaluated. Advantageously, the lamp voltage is thereby integrated over a whole period or over a multiple of a whole period of the lamp voltage and it is then determined that the rectifier effect is present if the integration result deviates from zero. If the detected lamp voltage or the parameter dependent thereon is superimposed with a d.c. component, then this d.c. componentxe2x80x94rather than zeroxe2x80x94is given as desired value for the integration result.
In practice, the presence of the rectifier effect is only determined upon if the integration result lies outside a predetermined desired value range. The reliability of the recognition of the rectifier effect can be further improved in that the presence of the rectifier effect is only determined upon if the integration results deviates from the predetermined desired value or from the predetermined desired value range a plurality of times in succession. This is sensible because the rectifier effect is a fault which appears insidiously so that in the recognition of the rectifier effect it must be ensured that the presence of a rectifier effect is not determined upon, and correspondingly reacted to, too hastily. It can thus be provided that the presence of the rectifier effect is only determined upon if the integration result deviates from the predetermined desired value, or from the predetermined desired value range, 32 times in succession each 255th period of the lamp voltage.
In accordance with a preferred exemplary embodiment of the present invention, the lamp voltagexe2x80x94or the parameter dependent thereonxe2x80x94is xe2x80x9cintegratedxe2x80x9d in that the duration of the positive half-wave of the detected parameter is compared with the duration of the negative half-wave so that the presence of a rectifier effect is then determined upon if the difference of the temporal durations of the positive and negative half-waves exceeds a predetermined tolerance value or tolerance range. In this case there can in particular be employed a counter which receives a reference timing signal and then upon zero crossing of the detected parameter is started, in order to count up or count down during the following half-period. When the detected parameter again reaches a zero crossing the counter begins to count in the opposite direction. Thus, in order not to determine upon the presence of the rectifier effect, the counter mustxe2x80x94after one period of the detected parameterxe2x80x94have again reached its initial count value, or its final count value must lie within a predetermined tolerance range in the vicinity of the initial count value.